Projection apparatuses that use a semiconductor laser as a light source have been spread under a background of high output performance and cost-down of semiconductor lasers, and improvement in performance of MEMS (Micro Electro Mechanical System) mirrors that are used to reflect a coherent light beam.
However, there is a problem in that coherent beams emitted from a coherent light source cause speckles to be generated on a projection screen. Speckles are a spotted pattern which is formed when a coherent light beam such as a laser beam is illuminated on a scattering plane. If speckles are generated on a screen, they are observed as spotted luminance unevenness, i.e. brightness unevenness, thus becoming a factor of giving a physiologically adverse effect on an observer. It is considered that the reason why speckles are generated in the case of using coherent light beams is that coherent light beams reflected from respective portions of a scattering and reflecting plane such as a screen have very high coherency so that the coherent light beams interfere with one another to generate speckles.
The inventor of the present invention invented in the past an illumination device that makes speckles inconspicuous in an illumination zone LZ when the illumination zone LZ is illuminated with coherent light beams, with an optical scanning device to scan an optical device with coherent light beams incident thereon, so that the optical device illuminates the illumination zone LZ with the coherent light beams overlapped one another thereon. The inventor also invented a projection apparatus that makes speckles generated on a screen inconspicuous when projecting an optically modulated image on the screen, using such a type of illumination device. (See Japanese Laid-Open Patent No. 2012-58481.)